This invention pertains to airbag canisters. More particularly, this invention pertains to airbag canisters or housings that are selectively heat treated to enhance ductility and toughness, and a method for making such canisters.
Airbags or supplemental restraint systems are an important safety feature in many of today""s vehicles. Airbag deployment technology uses controlled combustion or an xe2x80x9cexplosionxe2x80x9d to rapidly expand or deploy the airbag upon sensing an impact with another vehicle. This controlled explosion is contained within a canister or housing so that the rapidly expanding gases can be directed into the airbag for inflation.
Containing the controlled explosion of these chemicals is necessary for proper deployment of the airbag. The canister in which the chemicals are contained must be configured and manufactured to assure complete, controlled and predictable combustion within certain given parameters and requirements. One of these requirements is maintaining the structural integrity of the canister. That is, the canister must be configured and fabricated such that it maintains its integrity through the combustion process and subsequent airbag deployment. In testing of these canisters, in the event that a canister does rupture or yield, such rupturing or yielding must be predictable.
One currently used combustion chamber includes a substantially tubular member having two open ends. The chamber is formed from rolled and seal-welded plate stock, or is drawn as a seamless tube, such as that used for common piping. In this arrangement, however, welds are needed to seam-weld the rolled plate and/or seal-weld a plate to an open end of the tube. These welds are highly critical and as such require considerable labor and in certain instances testing to assure weld integrity throughout the combustion process and airbag deployment. It has been observed that these welds can crack or fail, thus, compromising the integrity of the canister, and possibly the operation of the airbag.
The canisters are tested to assure that they retain their structural integrity during airbag deployment. One such test is a burst test. This is a destructive-type test in which a canister is subjected to internal pressures significantly higher than those expected during normal operational use, i.e., airbag deployment. In this test, the canister is subjected to increasing internal pressures until failure.
In reviewing the burst test results and studying the test canister specimens from these tests, it has been found that failure occurs through ductile failure, brittle failure, and sometimes a combination of these two phenomena. It has been observed that in ductile fracture or failure an outturned rupture exemplified by an opened bulge (such as would be exhibited by a bursting bubble) occurs. This rupture is localized within a subject area. In a brittle fracture, on the other hand, through-wall longitudinal cracks along the length of the canister are exhibited which are indicative of a brittle zone in the material.
At times, a combination of these two failure mode can be observed. For example, a failure may occur due to ductile failure in which case a rupture or opened bulge is found. In those instances where a combination of the failure mechanisms is found, brittle cracks can propagate from the ductile, ruptured area.
Accordingly, there exists a need for an airbag canister having a high degree of structural integrity with a reduced number of welds. Preferably, such a canister is formed from relatively common carbon steel materials. Most preferably, such a canister is fabricated in a method using efficient and cost-effective parts and processes for manufacturing the canister.
A selectively heat treated canister for use in a vehicle airbag deployment system is configured to contain combustion materials and to contain gases produced from the combustion process. The canister includes a tubular body having a length and a longitudinal axis. The body is formed as at least two drawn sections. Preferably, the canister body is formed from a flat stock material that is drawn. Each prior section is drawn one more time than a successive adjacent section.
The canister defines a closed end and an open end. The closed end is at a least drawn section and the open end is at a most drawn section. The canister defines a heat treated region and is selectively heat treated at at least one transition zone between adjacent drawn sections. The combination of drawing and heat treating increases the toughness and ductility of the canister to reduce or eliminate the potential for unwanted failure modes in testing.
In a current embodiment, the canister body is formed as three drawn sections and defines a first worked zone that is drawn three times, a second worked zone that is drawn twice and a third worked zone that is drawn once. The heat treated region overlies a transition region between the second worked zone and the third worked zone.
Governmental specifications establish limitations for the material used for airbag canisters. One such limitation is that the canister material can have no more that 0.15 percent carbon. To this end, the canister can be formed from a low carbon steel material, such as AISI 1006 to 1010. The canister can also be formed from a high strength low alloy steel such as HSLA 50.
A method for making a combustion containing canister for use in an airbag deployment system includes the steps of providing a steel plate and drawing the plate. In a first drawing step, a first portion of the plate is drawn into a die in a first work step to define a first worked zone. The method further includes drawing, in a second drawing step, a second portion of the plate into the die in a second work step to define a second worked zone and to further draw the first worked zone.
Optionally, a third drawing step can be carried out in which a third portion of the plate is drawn into the die in a third work step to define a third worked zone, to further draw the second worked zone and the first worked zone, to define a canister body. This process can be carried out to form a canister body having a plurality of worked zones. A current method includes a first drawing step to define a first worked zone, a second drawing step to define a second worked zone and a third drawing step to define a third worked.
The method includes heat treating a portion of the canister body after working. Preferably, that portion of the canister body that is heat treated extends across a transition between worked zones. In a present method, the heat-treating step is carried out by induction heating, and the canister body is heat treated at a transition between the third worked zone and the second worked zone.
During burst testing, it was found that fracture of the canisters began in the least work hardened or lowest tensile strength zone and would propagate toward the end of the canister (i.e., through the work hardened zones). Heat treating at a transition zone between worked zones creates a region that absorbs the energy of the propagating crack that began in the weaker zone and stops crack propagation.
These and other features and advantages of the present invention will be apparent from the following detailed description, in conjunction with the accompanying drawings, and the appended claims.